Methods for synthesizing 2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine

ABSTRACT

2-Chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine is synthesized by reacting a 2-chloro-6-substituted purine with a protected and activated 2-deoxy-2-fluoro-D-arabinofiranose; and reacting with a base such as ammonia to provide 2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine. When the purine reactant is substituted in the 6 position with a halogen, a reaction step with an alkoxide is carried out prior to the reaction with ammonia.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 09/889,287filed Jul. 16, 2001, now U.S. Pat. No. 6,949,640, which is a NationalStage of PCT/US01/05320, filed Feb. 16, 2001 and claims priority fromApplication No. 60/183,422 filed Feb. 18, 2000.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with support under U.S. Government Grant No.P01CA 34200 awarded by the National Cancer Institute. The U.S.Government has certain non-commercial rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to methods for synthesizing achemotherapeutic agent that is useful in the treatment of variousmalignancies. More particularly, this invention relates to improvedmethods for synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-aminewherein the anionic form of a 2-chloro-6-substituted-purine is reactedwith a protected and activated 2-deoxy-2-fluoro-D-arabinofuranosefollowed by reacting with an appropriate base such as ammonia to provide2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine. Thepresent invention also relates to novel intermediates used insynthesizing the2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine suchas2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-6-alkoxy-9H-purinesand certain2-chloro-6-substituted-9-(2-deoxy-2-fluoro-3,5-diprotected-β-D-arabinofuranosyl)-9H-purines.

BACKGROUND OF THE INVENTION

Clofarabine[2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine]has exhibited cytotoxicity in mice inoculated with P388 leukemia. Asreported by Montgomery et al., Synthesis and Biologic Activity of2′-Fluoro-2-Halo Derivatives of 9-β-D-Arabinofuranosyladenine, Journalof Medicinal Chemistry, 1992, 35, pp. 397-401, clofarabine provided agood increase in life span of mice inoculated with P388 leukemia. The2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine wasthe most effective compound in the tested system. In addition, thiscompound exhibited reduced cleavage in vivo of the glycosidic bond ascompared to Fludarabine.

The reported method for synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-aminecomprises a procedure using3-O-acetyl-5-O-benzoyl-2-deoxy-2-fluoro-D-arabinofuranosyl bromide forthe coupling with 2,6-dichloropurine, followed by anamination/deprotection sequence. (See Montgomery, et al.,9-(2-Deoxy-2-fluoro-β-D-arabinofuranosyl)guanine: A Metabolically StableCytotoxic Analogue of 2′-Deoxyguanosine, Journal of Medicinal Chemistry,1986, 29, pp. 2389-2392; and Montgomery et al., Synthesis and BiologicActivity of 2′-Fluoro-2-halo Derivatives of9-β-D-Arabinofuranosyladenine, Journal of Medicinal Chemistry, 1992, 35,pp. 397-401).

However, the reported method for synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amineresulted in low overall yields of product, typically in the range ofabout 13%. The described coupling reaction produced a mixture ofnucleosides from which the desired 9-β intermediate was obtained in only32% yield after careful chromatography. Direct amination/deprotection ofthis material gave the desired2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine,plus a partially benzoylated2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine thatrequired further base treatment. Pure2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine wasobtained only after several recrystallizations to remove salts andresidual benzamide.

Such inefficient reactions will inhibit the ability to commerciallyproduce2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine.Thus, there is a need for an improved method for synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine thatresults in increased yields and/or reduced process steps.

SUMMARY OF THE INVENTION

One aspect of the present invention is to provide a relativelyhigh-yield method of synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine thatcomprises reacting the anionic form of a 2-chloro-6 substituted purinewith a protected and activated 2-deoxy-2-fluoro-D-arabinofuranose toprovide a 2,6-dichloro-9-substituted purine nucleoside. That product isthen reacted with an alkoxide to provide a 2-chloro-6-alkoxy purinenucleoside. That compound is then reacted with ammonia to provide2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine.

Another aspect of the present invention relates to a method forsynthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine byreacting the anionic form of a 2-chloro-6-substituted-purine with aprotected and activated 2-fluoro-2-deoxy-D-arabinofuranose to provide areaction product comprising a purine nucleoside, followed by reactingthe purine nucleoside with an appropriate base such as ammonia toprovide2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine.

The present invention also relates to novel intermediates used insynthesizing the2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine.These intermediates include2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-6-alkoxy-9H-purinesand2-chloro-6-substituted-9-(2-deoxy-2-fluoro-3,5-diprotected-β-D-arabinofuranosyl)-9H-purineswherein the 6-substituent is selected from the group selected fromamino, protected amino groups, azido and alkoxy.

Other features and objects and advantages of the present invention willbecome apparent from a reading of the following description.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of one embodiment of a reaction comprisinga synthesis method of the present invention.

FIG. 2 is a schematic diagram of an alternative embodiment of anothersynthesis method according to the present invention.

FIG. 3 is a schematic diagram of an alternative embodiment of a furthersynthesis method according to the present invention.

BEST AND VARIOUS MODES FOR CARRYING OUT THE PRESENT INVENTION

Reference to FIG. 1 illustrates one of the synthesis methods accordingto the present invention. This chemical reaction as illustrated in FIG.1 provides a convenient process for preparing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine(5)inthree steps and has resulted in overall yields of about 44%.

For the first step, a protected and activated2-deoxy-2-fluoro-D-arabinofuranose 2 is reacted with a2-chloro-6-substituted purine 1 to provide a reaction product comprisinga 9-substituted purine nucleoside 3. In this embodiment of the presentinvention, the preferred group in the 6 position is a halogen.

The preferred 2-chloro-6-substituted purine in the anionic form employedin this reaction scheme is 2,6-dichloropurine. Examples of suitableanionic forms include alkali metal salts, and organic amine salts.Alkali metal salts include sodium, potassium, and lithium salts. Themetal salts can be obtained from metal hydrides such as NaH, KH and LiHor alkoxides such as NaOCH₃ and KOCH₃.

Organic bases for forming amine salts include hindered strong aminebases such as DBU(1,8-diazabicyclo [5.4.0] undec-7-ene); DBN(1,5-diazabicyclo [4.3.0]non-5-ene); Dabco (1,4-diazabicyclo [2.2.2]octane); and N,N-diisopropylethylamine.

A preferred anionic form is the sodium salt. The anionic form is neededto achieve the desired coupling reaction.

The 2-deoxy-2-fluoro-D-arabinofuranose contains protecting groups on the3- and 5-hydroxyl groups and an activating group in the C-1 position.

Hydroxy protecting groups known in the art are described in Chapter 3 ofthe Protective Groups in Organic Chemistry, McOmie Ed., Plentum Press,New York (1973), and Chapter 2 of Protective Groups in OrganicSynthesis, Greene, T., John Wiley and Sons, New York (1981 and 1999);disclosures of which are incorporated herein by reference. Suitableprotecting groups for the hydroxyl groups include ester forming groups,carbonates, alkyl ethers, aryl ethers, silyl ethers and carbonates.Examples of suitable esters are formyl, acetyl, substituted acetyl,propionyl, butynyl, pivaloyl, 2-chloroacetyl, benzoyl, substitutedbenzoyl, phenoxycarbonyl, methoxyacetyl and toluoyl.

Examples of carbonate derivates are phenoxycarbonyl, ethoxycarbonyl,butoxycarbonyl, vinyloxycarbonyl, 2,2,2-trichloroethoxycarbonyl andbenzyloxycarbonyl.

Examples of alkyl ether and aryl ether forming groups are benzyl,p-chlorobenzyl, diphenylmethyl, triphenylmethyl, t-butyl, methoxymethyl,tetrahydropyranyl, allyl, tetrahydrothienyl, 2-methoxyethoxymethyl.

Examples of silyl ether forming groups are trialkysilyl, trimethylsilyl,isopropyldialkylsilyl, alkyldiisopropylsilyl, triisopropylsilyl,t-butyldialkylsilyl and 1,1,3,3-tetraisopropyldisiloxanyl.

Examples of carbamates are N-phenylcarbamate and N-imidazoylcarbamate.

Mixtures of protecting groups can be employed if desired. For example,the 2-deoxy-2-fluoro-D-arabinofuranose 2 may have either two acylgroups, two ether groups, or combinations of acyl and ether groups.

Examples of activating groups for the C-1 of the carbohydrate includehalogen such as Cl, Br and F; alkylsulfonyloxy, substitutedalkylsulfonyloxy; arylsulfonyloxy, and substituted arylsulfonyloxy.

Suitable alkyl substituents contain 1-8 carbon atoms and more typically1-4carbon atoms such as methyl, ethyl and propyl. A suitable aryl groupincludes phenyl.

Examples of alkyl- and substituted alkyl-sulfonyloxy groups aremethanesulfonyloxy and 2-chloroethanesulfonyloxy.

Examples of aryl- and substituted aryl-sulfonyloxy groups arebenzenesulfonyloxy, toluenesulfonyloxy, p-nitrobenzenesulfonyloxy andp-bromobenzenesulfonyloxy; while most preferred is methanesulfonyloxy.

A preferred protecting group is benzoyl and a preferred activating groupis bromine. A specific compound that may be used as sugar compound 2 is2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-D-arabinofuranosyl bromide asprepared by C. H. Tann, et al., J. Org. Chem., 1985, 50, 3644-3647; thedisclosure of which is hereby incorporated by reference.

The purine and activated carbohydrate derivative are typically employedin approximately equivalent amounts or with an excess of the purine andmore typically about 1:1 to about 3:1;preferably about 1:1 to about1.5:1 and more preferably about 1:1 to about 1.2:1 of purine toactivated carbohydrate derivative.

This step of the process is typically carried out at temperatures ofabout 0° to about 100° C., more typically about 20° C. to about 70° C.and preferably about 20° C. to about 40° C.; and at normal atmosphericpressures. However, higher or lower pressures can be employed ifdesired. This step of the process typically takes about 3 to about 24hours for completion.

The reaction of the above-described purine compound 1 with thearabinofuranose sugar 2 preferably takes place in the presence of asolvent. Such solvent may be a dipolar, aprotic solvent such as acetone,acetonitrile, dimethylformamide, dimethyl sulfoxide, sulfolane,dimethylacetamide and ethers such as tetrahydrofuran, dioxane, anddimethoxyethane.

When the reaction of the purine 1 and arabinofuranose sugar 2 iscomplete, the reaction mixture may be filtered and the solvent may beevaporated until a foam is obtained. The foam may be purified on flashsilica using isopropyl acetate/hexane or any other suitable solution asthe eluent. The fractions containing the desired 9-β isomer may becombined and evaporated to a residue that may then be crystallized fromethanol to give the desired 9-substituted purine nucleoside 3.

For the next step, the 9-substitued purine-nucleoside 3 is reacted withan alkoxide to provide the corresponding 2-chloro-6-alkoxy purinenucleoside 4. The alkoxide is preferably an alkali metal alkoxide andmost preferably sodium methoxide.

This step of the process is typically carried out at temperatures ofabout 0° C. to about 100° C., more typically about 20° C. to about 40°C., and at normal atmospheric pressures. Higher or lower pressures canbe employed, if desired. This step of the process typically takes about3 to about 24 hours for completion.

Moreover, this step of the process preferably takes place in thepresence of a solvent, with a preferred solvent being an alcohol whichcorresponds to the alkoxide used in the reaction. Upon completion, thereaction mixture may be treated with an ion exchange resin, filtered andevaporated to a residue. One commercially available ion exchange resinthat has proved useful for this purpose is Dowex 50WX8-400 ion-exchangeresin.

The desired 6-alkoxypurine nucleoside 4 may be derived from the residueobtained in this step by triturating the residue with hexane severaltimes, followed by decantation of the supernatant liquor. The residuethus obtained may then be either recrystallized, or slurried in coldisopropyl alcohol in lieu of recrystallization, to give 6-alkoxypurinenucleoside 4.

Finally,2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5)may be obtained by reacting the 6-alkoxypurine nucleoside 4 and ammonia.

This step of the process is typically carried out at temperatures ofabout 20° C. to about 120° C. and more typically about 70° C. to about100° C.; and typically at pressures generated in a sealed vessel at theabove temperatures. This step of the process typically takes about 12hours to about 24 hours for completion.

This step of the process can be carried out in the presence of analcoholic solvent such as methanol or ethanol or in the absence of asolvent.

The ammonia is typically present as an alcoholic solution such as inmethanol or ethanol (typically saturated at 5° C.). In a preferredembodiment, this reaction takes place in a stainless steel bomb at 80°C. (65 psi). When the reaction is completed, the solvent may be removedand the residue dissolved in refluxing methanol, and preferablyhot-filtered. Upon cooling, the crude product2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5)may be isolated by filtration. The product may be recrystallized frommethanol to give high-quality-2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine(5). Further recrystallizations of evaporated filtrates from methanolare optional to obtain additional2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5).

Reference to FIG. 2 illustrates another reaction scheme according to thepresent invention for synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5).

In the first step, a protected and activated2-deoxy-2-fluoro-D-arabinofuranose 2 is reacted with the anionic form ofa 2-chloro-6-substituted purine 1 to provide a reaction productcomprising a 9-substituted purine nucleoside 3. Examples of suitablesubstituents in the 6 position include groups such as amino, protectedamino groups, azido and alkoxy, with amino being preferred. Suitablealkoxy groups are methoxy and ethoxy.

Suitable amino protecting groups are acyl, imino, and carbamates.Suitable acyl groups are acetyl-, benzoyl-, p-methoxybenzoyl,2-methylbutryl- and pivaloyl.

A suitable imino group is dimethylaminomethylene.

Suitable carbamates are isobutyl-, t-butyl-, benzyl-, p-methoxybenzyl-,carbamates.

The preferred purine is the anionic form of 2-chloro-6-aminopurine.Examples of suitable anionic forms include alkali metal salts andorganic amine salts as discussed above in the first embodiment of thepresent invention. Preferred anionic forms are the sodium salt and aminesalts such as DBU.

The 2-deoxy-2-fluoro-D-arabinofuranosyl moiety contains protectinggroups on the 3- and 5- hydroxyl groups and an activating group in theC-1 position.

Examples of suitable protecting groups and activating groups are thosediscussed above for the first embodiment according to the presentinvention.

A preferred protecting group is benzoyl and a preferred activating groupis bromine.

A specific compound that may be used as the sugar reactant 2 is2-deoxy-2-fluoro-3,5-di-O-benzoyl-2-α-D-arabinofuranosyl bromide.

The purine and the activated carbohydrate derivative are typicallyemployed in approximately equivalent amounts or with an excess of thepurine and more typically about 1:1 to about 3:1, preferably about 1:1to about 1.5:1 and more preferably about 1:1 to about 1.2:1 of purine tothe activated carbohydrate derivative.

This step of the process is typically carried out at temperatures ofabout 0° C. to about 100° C., more typically about 20° C. to about 70°C. and preferably about 20° C. to about 40° C.; and at normalatmospheric pressures. However, higher or lower pressures can beemployed if desired. This step of the process typically takes about 3 toabout 96 hours for completion.

The reaction of the above-described purine compound 1 with thearabinofuranose sugar 2 preferably takes place in the presence of asolvent. Such solvent may be a dipolar, aprotic solvent such as acetone,acetonitrile, dimethylformamide, dimethyl sulfoxide, sulfolane,dimethylacetamide, and ethers such as tetrahydrofuran, dioxane anddimethoxyethane.

Upon completion of the reaction of the purine 1 and arabinofuranosesugar 2, the reaction mixture may be filtered and the solvent may beevaporated until a foam is obtained. The foam may be purified on flashsilica using isopropyl acetate/hexane or any other suitable solution ofthe eluent. The fractions containing the desired 9-β isomer may becombined and evaporated to a residue that may then be recrystallizedfrom ethanol to give the desired 9-substituted purine nucleoside 3.

When the group in the 6 position is amino or a protected amino group,the desired 2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5) may be obtained by reacting the purine nucleoside 3 and abase such as ammonia and/or an alkali metal alkoxide such as sodiummethoxide, an alkali metal carbonate such as sodium carbonate, and aalkali metal hydroxide such as lithium hydroxide. This step of theprocess with these groups is typically carried out at temperatures ofabout −20° C. to about 80° C. and more typically about 0° C. to about50° C.; and typically at pressures generated in a sealed vessel at theabove temperatures. This step of the process typically takes about 1hour to about 24 hours for completion.

When the group in the 6 position is azido, the desired2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine(5)may be obtained by reacting the purine nucleoside 3A with a reducingagent such as a hydrogenating agent to reduce the azido group to anamino group and then reacting with a base as discussed above (see FIG.3). The reducing step can be carried out, for instance, by reacting withhydrogen in the presence of a hydrogenation catalyst such as platinum orpalladium. This step of the process is typically carried out at aboutnormal room temperatures and a pressure of about 1 atm to about 3 atm.Moreover, when the group in the 6 position is azido, the order of thereaction steps can be reversed. In particular, the desired2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine(5)may also be obtained by reacting the purine nucleoside 3 with a base asdiscussed above and then reacting with a reducing agent to reduce theazido group to an amino group.

When the group in the 6 position is alkoxy, the desired2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5)may be obtained by reacting the purine nucleoside 3 with ammonia.

This step of the process is typically carried out at about normal roomtemperatures to about 120° C. and more typically about 70° C. to about100° C.; and typically at pressures generated in a sealed vessel at theabove temperatures. This step of the process typically takes about 12hours to about 24 hours for completion.

This step of the process is preferably carried out in the presence of analcoholic solvent, such as methanol or ethanol or in the absence of asolvent.

Preferred embodiments for converting 3 to 5 include using ammonia orsodium methoxide. In the examples where the group (R in 3 in FIG. 2) isamino, protected amino such as acylamino, imino, and carbamate, onepreferred embodiment is to use sodium methoxide at about 0° C. to normalroom temperatures. Alternatively, ammonia can be used.

The ammonia, when used, is typically present as an alcoholic solutionsuch as in methanol or ethanol (typically saturated at 5° C.). In apreferred embodiment, this reaction takes place in a stainless steelbomb at room temperature. When the reaction is completed, the solventmay be removed and the residue dissolved in refluxing methanol, andpreferably hot-filtered. Upon cooling, the crude product2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5)may be isolated by filtration. The product may be recrystallized frommethanol to give high-quality2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5).Further recrystallizations of the evaporated filtrates from methanol areoptional to obtain additional2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine (5).

One of ordinary skill in the art will readily see that some modificationmay be made to the preferred embodiments of the present invention setforth above. Further illustration of the present invention is set forthin the following examples, which are not to be construed as limiting theinvention in any manner. The examples illustrate the individual steps ofthe above-described invention process.

EXAMPLE 12,6-dichloro-9-(2-deoxy-2-fluoro-3,5-di-O-benzoyl-β-D-arabinofuranosyl)-9H-purine(3)(FIG. 1. R¹=Cl)

A suspension of 2,6-dichloropurine (4.0 g, 21.2 mmol) in anhydrousacetonitrile (130 ml) at room temperature was treated with NaH (916 mgof 60% in oil washed with heptane, 22.9 mmol), and the mixture wasstirred 15 minutes under argon. To this stirred suspension, a solutionof 2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-D-arabinofuranosyl bromide (C. H.Tann, et al., J. Org. Chem., 1985, 50, 3644-3647; 9 g, 21.3 mmol) inacetonitrile (29 ml) was added dropwise, and the mixture was stirred atroom temperature overnight. Insoluble material was removed by filtrationand washed with acetonitrile and chloroform. The combined filtrate andwashings were evaporated to a near glass. A chloroform solution of thisresidue was applied to a flash column containing silica gel 60 (70-230mesh, E. Merck). Elution with chloroform provided pure fractions thatwere combined and crystallized from boiling ethanol to give 1.6 g (14%)pure product (HPLC, 100% 9-β). Material from less pure fractions wascrystallized from chloroform at 5° C., 6.3 g (56%) (HPLC, 97% 9-β, 3%9-α). After structure confirmation by ¹H NMR, this material was useddirectly in the next step.

EXAMPLE 22-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-6-methoxy-9H-purine(4)

2,6-Dichloro-9-(2-deoxy-2-fluoro-3,5-di-O-benzoyl-β-D-arabinofuranosyl)-9H-purine(3)(4.7 g, 8.85 mmol) prepared as in Example 1 was suspended in anhydrousmethanol (200 ml) at room temperature. To this mixture, a 25 wt %solution of sodium methoxide in methanol (2.23 ml, 9.75 mmol) was addedin one portion. After being stirred for 1.5 hours, the reaction became aclear solution. After 20 hours, the reaction was neutralized with astrong cation exchange resin (Dowex 50×4 [H⁺]) which was collected after15 minutes and washed with methanol. The combined filtrate was subjectedto evaporation and was led to provide a gum that was triturated with twoportions of petroleum ether 30-60° C. (decanted). The remaining materialwas dissolved in hot 2-propanol (30 ml)(filtered to clarity), and thesolution was allowed to deposit crystals at room temperature beforebeing chilled (5° C.) overnight. The product was collected, washed withice-cold 2-propanol, and dried in vacuo to give the title compound, 1.7g (60%), mp 199-200° C. (HPLC, 98%). Flash chromatography of theevaporated filtrate (silica gel 60, 70-230 mesh, E. Merck) with 95.5chloroform-methanol as solvent provided additional material, 0.55 g(20%), mp 196-197° C. (HPLC, 96%).

EXAMPLE 32-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine(5)

2-Chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-6-methoxy-9H-purine(4)(6.2 g, 19.5 mmol) prepared as in Example 2 was placed in a stainlesssteel pressure bomb with 300 ml ethanol saturated (0° C.) with anhydrousammonia. The sealed vessel was heated at 80° C. for 16 hours. Moreethanolic ammonia (30 ml) was added to the incomplete reaction, andheating was continued for 4 hours. The reaction solution containing atrace of starting material was evaporated to a white foam thatcrystallized from hot methanol (75 ml), 5.1 g. This relatively purematerial was dissolved in refluxing methanol (110 ml), filtered, allowedto cool to room temperature, then chilled. Pure title compound wasobtained in two crops, total 4.6 g (78%), mp 231° C. (HPLC, 99%).

EXAMPLE 42-chloro-9-(2-deoxy-2-fluoro-3,5-di-O-benzoyl-β-D-arabinofuranosyl)-9H-purin-6-amine(3) (FIG. 2, R=amino)

A suspension of 2-chloroadenine (21 mg, 0.12 mmol) in anhydrousacetonitrile (2.5 ml) at room temperature was treated dropwise with 98%DBU (18 μl, 0.12 mmol), and the mixture was stirred 25 minutes underargon. To this stirred suspension, a solution of2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-D-arabinofuranosyl bromide (2) (48mg, 0.1 mmol) in acetonitrile (0.8 ml) was added dropwise. The mixturewas stirred at room temperature until the2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-D-arabinofuranosyl bromide (2) wasconsumed. After 96 hours, insoluble material was removed by filtrationand washed with CHCl₃. The combined filtrates were evaporated to aresidue that was dissolved in CHCl₃. This solution was applied to apreparative layer silica gel plate (Analtech, 10×20 cm, 1,000 microns)that was developed twice in 97:3 CHCl₃/MeOH. Product bands wereextracted with 1:1 CHCl₃/MeOH, and the extracts were evaporated to givewhite solids, 16 mg (28%) (HPLC, 100% 9-β) and 20 mg (34%) (HPLC, 97%9-α).

The above description and examples of the present invention are notintended to be limiting, and it is recognized that one of skill in theart will readily discern variations of this description, that areintended to be included within the spirit and scope of the invention.

1. A method for synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-aminewhich comprises: a) reacting the anionic form of a2-chloro-6-substituted purine with a protected and activated2-deoxy-2-fluoro-D-arabinofuranose; wherein the 6-substituted group inthe 2-chloro-6-substituted purine is selected from the group consistingof amino and protected amino; and then (b) reacting with ammonia toprovide the2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine. 2.The method of claim 1 wherein the 6-substituted group in the2-chloro-6-substitute purine is amino.
 3. The method of claim 1 whereinthe anionic form is an alkali metal salt or organic amine salt.
 4. Themethod of claim 1 wherein the anionic form is an alkali metal salt. 5.The method of claim 4, wherein the alkali metal is sodium.
 6. The methodof claim 1 wherein the anionic form is an organic amine salt.
 7. Themethod of claim 6, wherein the organic amine salt is1.8-diazabicyclo[5.4.0]undec-7-ene.
 8. The method of claim 1 wherein theprotecting group on the 3- and 5-hydroxyls of the2-deoxy-2-fluoro-D-arabinofuranose is selected from the group consistingof an acyl group, ether group, and combinations thereof, and wherein theactivating group at C-1 of the 2-deoxy-2-fluoro-D-arabinofuranose isselected from the group consisting of halo, alkylsulfonyloxy, andarylsulfonyl groups.
 9. The method of claim 1 wherein the2-deoxy-2-fluoro-D- arabinofuranose is2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-D-arabinofuranose bromide.
 10. Themethod of claim 1 wherein the reaction of the 2-chloro-6- substitutepurine with the 2-deoxy-2-fluoro-D-arabinofuranose takes place in thepresence of a dipolar, aprotic solvent.
 11. The method of claim 10wherein the solvent is selected from the group consisting of acetone,acetonitrile, dimethylformamide, dimethyl sulfoxide, sulfolane,dimethylacetamide, and an ether.
 12. The method of claim 1 wherein step(b) takes place in the presence of a solvent.
 13. The method of claim 12wherein the solvent is an alcohol.
 14. The method of claim 1 wherein theammonia is present as an alcoholic solution.
 15. The method of claim 14wherein the alcoholic solution is in methanol or ethanol.
 16. A methodfor synthesizing2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-aminewhich comprises: a) reacting the anionic form of a2-chloro-6-substituted purine with a protected and activated2-deoxy-2-fluoro-D-arabinofuranose; wherein the substituted group isamino or a protected amino; and then (b) reacting with a base to providethe 2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)-9H-purin-6-amine.17. The method of claim 16 wherein the base is an alkali metal alkoxide.